This invention relates to the use of gaseous nitrogen dioxide or nitric oxide to inhibit the vinyl polymerization of a 2-isocyanatoalkyl ester of an .alpha.,.beta.-ethylenically unsaturated carboxylic acid.
Certain conventional polymerization inhibitors have been used to inhibit during distillation the vinyl polymerization of a 2-isocyanatoalkyl ester of an .alpha.,.beta.-ethylenically unsaturated carboxylic acid. For example, European Patent Office Application No. 78100156.5, Publication No. 144, published Jan. 10, 1979, discloses the use of phenothiazine to inhibit the polymerization of 2-isocyanatoethyl methacrylate during distillation.
While conventional polymerization inhibitors are generally effective in inhibiting polymerization of 2-isocyanatoalkyl esters during storage, they have not proven as effective during the distillation of the isocyanatoalkyl ester. In particular, these isocyanatoalkyl esters are susceptible to the formation of a hard, brittle, highly cross-linked polymer, which is referred to as "popcorn polymer" in the prior art because of its physical appearance. This popcorn polymer is especially deleterious, because once formed it tends to initiate further polymerization.
Relatively volatile inhibitors, such as p-methoxyphenol, have been employed during the distillation of 2-isocyanatoalkyl methacrylate to inhibit polymer formation in the gas phase and in the distillate. However, these volatile inhibitors have been observed to be relatively ineffective as polymerization inhibitors in this application.
Nitrogen oxides are known in the art to inhibit the polymerization of certain unsaturated compounds. U.S. Pat. No. 3,964,978 discloses the distillation of vinyl aromatic compounds in the presence of nitrogen dioxide to inhibit polymerization. U.S. Pat. No. 3,964,979 teaches the use of nitric oxide to inhibit the polymerization of vinyl aromatic compounds during distillation. British Patent No. 1,265,419 teaches that acrylic acid can be distilled in the presence of nitric oxide in the gas phase and phenothiazine in the liquid phase to minimize polymerization. However, the presence of nitrogen dioxide is taught to promote the polymerization of acrylic acid and to discolor the distillate.
Nitric oxide is disclosed by J. F. Villa and H. B. Powell, Syn. React. Inorg. Metal-Org. Chem., 6, pp. 59-63 (1976), to catalyze the trimerization of certain aliphatic isocyanates. This disclosure suggests that nitric oxide would not be suitable as a polymerization inhibitor for a 2-isocyanatoalkyl ester.